Pistons in internal combustion engines have long incorporated piston rings to improve performance. Piston rings reduce blow-by of gases during the compression and power strokes and allow lubricating oil or other lubricant to coat the cylinder wall but prevent excessive lubricating oil from entering the combustion chamber.
Many types of piston rings have been developed having differently contoured outer peripheries. For example, piston rings may have a taper face, a barrel face, or a plain face. The piston ring, commonly made from iron, may also have a porous coating, film, outer layer or surface to pick up oil for forming a lubricant film as it glides against the engine cylinder wall.
The piston ring construction may have the ring being substantially planar or it may incorporate a negative or positive twist in its relaxed position. The shape or twist of the piston ring, during operation of the piston ring, may change during the down strokes and the upward strokes for providing wiping of the lubricant during the down stroke and bypassing the lubricant film during the upstroke.
Lubrication regimes always balance the need to provide as low a viscosity of the lubricant as possible to decrease the inherent friction of the lubricant but also provide the needed high viscosity near the top dead center position of the power stroke to reduce mixed/boundary lubrication regimes. Not only is it recognized that the lubricant film thickness may vary and different lubrication requirements may be needed during different crank angles of the engine, but different circumferential positions around the piston may have different lubrication requirements particularly if there is a distorted cylinder bore, the piston has a tilt, or the piston and piston ring are not truly cylindrical.
What is need is an improved piston ring that provides for a cavity that absorbs and stores lubricant when excess lubricant is on the engine cylinder wall and can provide additional lubricant during the power stroke when more lubrication is needed.